The present invention generally relates to the use of anhydrous borate materials in subterranean treatment operations, and, more specifically, to cleanup fluids for affecting the removal of anhydrous borate materials from a formation after a subterranean treatment operation.
Treatment fluids can be used in a variety of subterranean operations. Such subterranean operations can include, without limitation, drilling operations, stimulation operations, production operations, remediation operations, sand control treatments and the like. As used herein, the terms “treat,” “treatment” and “treating” refer to any subterranean operation that uses a fluid in conjunction with achieving a desired function and/or for a desired purpose. Use of these terms does not imply any particular action by the treatment fluid. Illustrative treatment operations can include, for example, fracturing operations, gravel packing operations, acidizing treatments, scale dissolution and removal, consolidation treatments, and the like.
In the course of performing treatment operations in a subterranean formation, it can sometimes become necessary to block or divert the flow of certain fluids in the subterranean formation. In certain cases, it can sometimes be beneficial to divert the flow of a treatment fluid from a first region of the subterranean formation to a second region of the subterranean formation. For example, it can sometimes be desirable to divert a treatment fluid from a more permeable region of the subterranean formation to a less permeable region in which treatment is needed. In other cases, it can sometimes be desirable to temporarily seal fractures present within the subterranean formation to prevent fluid loss to the formation due to leak-off while a treatment operation is taking place. In still other cases, it can sometimes be desirable to seal the wellbore entirely for some period of time.
In most instances where a fluid is blocked or diverted in a subterranean formation during a treatment operation, it is desirable for the blockage or diversion to persist only temporarily, after which time production can resume. Temporary fluid blocking or diversion can also be desirable when a subterranean region that was formerly blocked needs to be subsequently treated with a treatment fluid to enhance its production. A number of substances have been used to temporarily block or divert fluid flow within a subterranean formation. Relative permeability modifiers can be used to block the flow of some fluids (e.g., an aqueous fluid) while not substantially inhibiting the flow of others (e.g., oil). Polymer gels can also be deposited within a subterranean formation to divert or block fluid flow therein. Once the gel has performed its desired function within the subterranean formation, the gel can be broken using various polymer breakers. Degradable polymers (e.g., polylactic acid) that degrade under the formation conditions and do not require the use of an external breaker can be used as well. A frequently encountered issue with polymer gels is that they can leave behind a gel residue on the surfaces of the subterranean formation, which oftentimes needs to be removed in a subsequent cleanup operation in order to restore the wellbore to full production.
Anhydrous borates are another class of materials that can be used for fluid blocking or diversion in subterranean formations. As used herein, the terms “anhydrous borates,” “anhydrous borate compounds” and “anhydrous borate materials” will equivalently refer to borate materials lacking a water of hydration in its molecular structure. Anhydrous borates are relatively insoluble in water in their dehydrated state but slowly re-acquire water(s) of hydration over time. Once hydrated, the borate materials can become soluble in an aqueous fluid. Depending on the temperature and other conditions present in a subterranean formation, the time required for anhydrous borates to become soluble can range between about 8 hours and about 72 hours, or even longer. During the time period when the borate material is insoluble (e.g., in particulate form), treatment operations can be conducted in which the anhydrous borates divert or block fluid flow within the subterranean formation. Once treatment operations have been completed, it can sometimes be necessary to wait for the anhydrous borates to become at least partially soluble before resuming production.
Partially dehydrated borate materials can also be used for fluid blocking or diversion in a subterranean formation. As used herein, the term “partially dehydrated borate materials” will refer to hydrated borate materials that contain a single water of hydration, when other more hydrated forms of the borate material are known to exist. For example, sodium tetraborate (borax) can have several hydrated forms including, for example, a monohydrate, a pentahydrate and a decahydrate. In this example, the monohydrate would be considered a partially dehydrated borate material, since higher hydrated forms exist. Like anhydrous borates, partially dehydrated borate materials can become slowly soluble in an aqueous fluid. However, partially dehydrated borate materials generally can become soluble faster than do anhydrous borates, typically within the range of about 15 minutes to about 1 hour. Although this time period can be sufficient to conduct certain treatment operations, partially dehydrated borate materials do not typically offer the greater timing flexibility of anhydrous borates for conducting treatment operations.